Cause and effect. Because many rides are not practical to get to by train. If you adjusted the start time, you might get a lot more travelling by train.
And depends on what sort of riders you are targetting. Is it just club riders with families?
I run my event on a Saturday, there are trains available, and it's still 2% (which means 2 riders). I would have no hesitation to move it to a sunday, it would even solve some park run related hassle on part of the route.
Anything up to 200 km and you are targeting club riders with families, unless you are happy with small numbers. But with fixed costs in the region of 200 pounds, I can't operate with fewer than 50 riders.
Anything above 300 and you are pretty much solely targeting AUK members, which come in all sorts of ages, but then you are bound to Saturdays, for obvious reasons